Wire wound potentiometers with printed circuit terminals



Sept. 22, 1970 G. DION 3,530,421

:WIRE WOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Filed Oct. 9,1968 5 Sheets-Sheet 1 INVENTOR. Geo fies Dion 'IATTORNEY G. DION3,530,421

Sept. 22, 1970 WIRE WOUND POTENTIOHETERS WITH PRINTED CIRCUIT TERMINALS3 Sheets-Sheet 2 Filed Oct. 9, 1968 m2 32 2o 4O 22 66 Sept. 22,1970 6.DION 3,530,421

WIRE WOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Filed Oct. 9,1968 3 Sheets-Sheet 3 FIG.4

I I I l 160 156 166 174 176 144 United States Patent 3,530,421 WIREWOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Georges Dion, NewYork, N.Y., assignor to Societe dlnstrumentation Schlumberger, acorporation of US. Cl. 338-174 6 Claims ABSTRACT OF THE DISCLOSURE Theends of a potentiometer resistance element, and especially a resistancecoil, are soldered to the potentiometer output terminals without havingbeen previously unwound. The electrical connection is effected byprinted circuits formed on an insulating substrate. Each printed circuitincludes a contact strip which underlies a portion of the ditferent endsof the resistance element, an adjoining junction strip which terminatesat each output terminal and a heating plate of relatively large surfacearea designed to support at least the tip of a conventional solderingtool. The heating plate is formed contiguous the two strips but isspaced from each output terminal a distance great enough to prevent thedisconnection of a soldered connection between a terminal and a junctionstrip when solder is applied to the contact strip. Preferably, thejunction strip is relatively flat and narrow and while providing goodelectrical conduction between it and the contact strip, providesrelatively poor heat conduction from a heating plate to an associatedsoldered output terminal.

The invention relates to potentiometers and more particularly, toprecision potentiometers which utilize a helical resistance element.

Potentiometers of the type presently under consideration typicallycomprise a housing, a resistance element in said housing and having theends thereof connected to output terminals and a wiper contact whichmoves on and along the resistance element. The resistance element iscomprised of a resistive wire wound helically on an insulating core ofcircular cross section. This core may be straight or circular dependingupon whether the potentiometer is of a linear or of a circular type.

In manufacturing this type of potentiometer a problem often arises inobtaining a satisfactory attachment of the helical resistive element tothe potentiometer output terminals. In order' to obtain an electricalsignal proportional, in absolute value, to the distance or angle of movement of the wiper on the resistance element, the distance, or the angle,between the two end turns of the resistance wire must be very accuratelyestablished. In prior art potentiometers the extremities of theresistive wire are unwound until an unwound wire portion is obtainedwhich has a length suflicient to permit the wire extremities to besoldered or otherwise connected to the potentiometer output terminals.This operation, which usually must be performed on resistance wireshaving very small diameters, is a delicate, tedious and costlyproposition. Moreover, since the lengths of unwound wire are notcontacted by the wiper and hence do not serve as a useful length of theresistance element, such lengths of unwound wire becomes sources ofresidual resistances which often introduce not readily determinable butnevertheless significant errors into measurements made with thepotentiometer.

It is an object of this invention to provide a potentiometer including ahelical resistance element, in which the potentiometer output terminalscan be readily and inexpensively connected to the extremities of theresistance element.

Another object of the invention is to provide a potentiometer whichincludes a helical resistance element wherein the useful length of theresistance element is very accurately established and wherein anyresidual resistance at the ends of the resistance element is of very lowmagnitude.

In accordance with one embodiment of this invention, a potentiometerembodies a helical resistance element along which a wiper can be movedand includes an insulating support member having one face thereofmounted adjacent the resistance element. This one face of the supporthas printed circuits formed thereon and at least two of these circuitsare formed into ribbon-like conductive strips hereinafter referred to ascontact strips which extend under portions of the resistance elementends for the purpose of making electrical contact therewith. Usingconventional printed circuit techniques, the edges defining the ends ofthe contact strips can be printed onto the insulating support veryaccurately. Hence, the useful length of the resistance element,considered with reference to these edges of the contact strips may alsobe determined with a high degree of accuracy. Each of the contact stripsmay be soldered to a corresponding portion of a resistance element endby applying molten solder to a more remote portion of the contact stripsand then relying upon capillary action to etlect solder flowage over thesurface of the strip to the coils of the resistance element. Each of theaforementioned two printed circuits further comprises a heating areaconnected to an output or terminal pin by a junction strip of relativelynarrow width. The junction strip introduces thermal losses and delays inheat conduction to the contact strips soldered to the resistance elementand therefore an output lead can be soldered to a printed output contactwithout fear of the resistance element becoming unsoldered from acorresponding contact strip.

The features and advantages of the invention will be best understood bythe following detailed description when taken in conjunction with theaccompanying drawings on which:

'FIG. 1 is a sectional side view of a circular or rotary potentiometerconstructed in acocrdance with the invention,

FIG. 2 is an enlarged section view of a part of the potentiometer ofFIG. 1,

FIG. 3 is a plan view of a portion of FIG. 2,

FIG. 4 is a plan view of a particular embodiment of an insulatingsupport on which the resistance element of the potentiometer may bemounted,

FIG. is a plan view of another embodiment of the insulating support-ofFIG. 4,

FIG. 6 is a view of a third embodiment of an insulating support for aresistance element which can be used for a linear potentiometer.

Referring now to FIG. 1, a rotary potentiometer constructed inaccordance with this invention is designated by the numeral 1, andincludes a base 2 of overall cylindrical shape and a circular cover 4suitably attached to the base 2 by, for example, screws 5. A cylindricalcontrol shaft 6 is mounted for rotation on coaxial ball bearings 8 and10 having their outer races mounted stationary in circular recessesformed in the base 2 and the cover 4, respectively.

An insulating sleeve 12 is mounted with an interference fit on the shaft6 and, in turn, has an electrically conductive wiper 14 fixedly securedthereto. The wiper 14 includes two resilient metal contact arms 16 and18 of different lengths, the longer arm 16 resiliently contacts acircular resistance element 20 and the shorter arm 18 is in wipingelectrical contact with a collector ring 22 mounted in concentricalignment with the circular element 20. Extending radially from the base2 are three insulating sleeves, one such sleeve being indicated at 26.Each sleeve receives an output conductor or lead such as 28 which passesthrough the wall of base 2. The three output conductors, such as 28, areindividually connected to the extremities of the resistance element 20and to the collector ring 22, respectively, by means of printed circuitswhich will be described in greater detail hereafter.

The resistance element 20 is mounted on an insulating support plate ordisk 30, as seen more clearly in FIGS. 2 and 3, and is comprised of aresistance wire 32 wound helically on an insulating core 34 of circularcross-sectional shape. The wire 32 may be an enamelled copper wire, forexample, and the core is typically obtained by helically winding thewire 32 on a straight, cylindrical mandrel which is then bent into acircular shape. During this latter operation, the end turns of the wire32 are made to adhere to the core by applying an adhesive, for examplevarnish, between certain of the turns of the wire and the mandrel. Theinsulating support 30 is of overall circular shape and has a circularaperture 36 formed centrally therein.

Printed circuits 38 are formed on the flat, internal surface of thesupport 30 by utilizing, for example, conventional photo-engravingtechniques. The resistance element 20 is surrounded by an annular ring40, composed of a suitable insulating material, such as Teflon. Thesupport 30 rests on an internal circular shoulder formed on the base 2.The assembly formed by the support 30, the insulating ring 40 and thewound resistance element 20 is held stationary in the base 2 by aretaining ring 24 having an inwardly curving tip 2411 obtained byinitially cutting a substantially V shaped groove in the internalsurface 27 of the sidewall of the base 2 and then forcing the taperingend of the thusly formed ring 24 downwardly and inwardly of the base 2.The inwardly bent or wedged tip 24a forces the adjoining portion of thering 40 radially inward and downward against the component 20 and thecomponent is thus clamped tightly against the printed circuit 38.

The printed circuits engraved on the insulating support are shaped asillustrated by FIG. 3. Two circuits 46 and 48 are located symmetricallyrelative to a radius 50 taken through the support 30 perpendicular tothe axis of rotation of the control shaft 6. Each of these circuits maybe described as comprising a narrow, elongated or ribbon-like contactstrip 52 extending approximately parallel to the diameter 50, a heatingarea 54, a narrow, elongated or ribbon-like junction strip 56 and anoutput contact 58. The outer contact end of the strip 52 extendsradially to a postion beneath the resistance element 20 and is tangentto one or several coils of the resistance wire, depending upon the widthof the contact strip. The contact strip is then electrically connectedto one or more turns of the resistance wire 32 by solder 60. The widthand spacing of these contact ends of the strip 52 can be defined withgreat accuracy utilizing conventional photoengraving techniques. Apotentiometer is thus obtained in which the useful electrical travel,limited by the blade width of the wiper, is very accurate. Furthermore,the residual resistance at the two extremities, that is to say theresistance between the contact arm 16 and one of the potentiometeroutputs, when the wiper is in its initial position (that is, in contactwith the first coil of the resistance wire) is less than the resistanceof one half of a turn of the resistance wire. The dimensions of theheating area 54 are large enough so that the area 54 can completelyaccommodate a conventional soldering iron placed thereon, therectangular shape of this surface being merely illustrative of a'configuration which conforms to the shape of a conventional rectangularsoldering iron tip. The length and width dimensions of the junctionstrip 56 are selected so as to offer relatively low electricalresistance but relatively high resistance to heat transfer between theoutput contact 58 and the area 54. This junction strip consists,preferably, of two straight, narrow portions 56a and 56b oriented atright angles with respect to each other so as to occupy as little spaceas possible.

The output contact 58, which is of generally circular shape, includes acircular aperture '62 at its center which extends perpendicularlythrough the support 30. The diameter of the aperture is large enough toreceive one of the conductors 28 (FIG. 1) which is inserted through theaperture 62 for soldering onto the contact 58.

A third printed circuit 64 of annular shape is also formed on the base30 so as to form the circular collector ring 22 (FIG. 1). The differentprinted circuits may be made as a continuous pattern to enable theelectrolytic application of a protective layer and then separated fromeach other by cutting an annular groove 66 through the conductive layerof the printed circuit.

To mount the resistance component 20 into the potentiometer, theinsulating support 30, the insulating ring 40 and the resistancecomponent 20 are first positioned in the base 2 with the air gap betweenthe two ends of the component 20 approximately midway between the twospaced apart contact ends 52. The upper extremity of the ring 24 is thenwedged inwardly causing the insulating ring 40 to clamp the resistanceelement 20 against displacement. To solder the spaced apart ends of theresistance component 20 to the ends of the contact strips 52, asoldering iron is placed upon the heating areas 54 and solder is placedon both of those portions of the contact strips 52 which are slightlyinwardly of the resistance element. The liquid solder then flows bycapillarity only over the surface of the contact and ultimately flowsinto the area between the extremities of the contact strips and theresistance wire. It is then possible to solder conductors 28 to outputcontacts 58 without accidentally unsoldering the resistance Wire becausethe thin, ribbon-like strips 56 and 52 offer a high thermal resistanceto the transfer of heat between the contacts 58 and the outerextremities of the strips 52.

FIG. 4 illustrates another embodiment of this invention which enables afixed potential to be taken ofi at a predetermined point along thelength of the helical resistance element. In this embodiment, theinsulating support disk is designated by the numeral and has fourprinted circuits formed thereon. The first three printed circuitsdesignated 102, 104 and 106, respectively, are identical to thoseillustrated by FIG. 3 and described hereinabove. The fourth circuit 108,like circuits 102 and 104, comprises a contact end 110, a heating area112, a junction strip 114 and an output contact 116. The contact end isangularly displaced from the contact end of circuit 104- by apredetermined angle, for example, 30 degrees and is also soldered ontothe resistive element in the manner described hereinabove. Thus, a fixedintermediate potential take-off is available which can be connected toan additional output terminal on the potentiometer. It is also possibleto connect, with a shunting resistance, contact 116 to one or the otherof the extremities of the resistance element. The resistance elementcan, of course, be divided into sections by several other printedcircuits thus making it possible to obtain an accurate stepping switchor a plurality of individual voltage sources.

FIG. illustrates another embodiment of this invention. In thisembodiment, the insulating disk is referred to by the numeral 118 andhas a circuit 120 printed thereon which includes a contact end 122 thatcovers relatively large, continuous arcuate conductive surface on whichthe resistive element is soldered. This contact end thus forms acontinuous thermal zone with its associated heating plate. Thisarrangement makes possible the provision of a zone or region of constantpotential at the corresponding end of the resistance element.

The instant invention, in addition to having application to circularpotentiometers is also applicable to linear potentiometers. FIG. 6illustrates an application of the instant invention to a linearpotentiometer which comprises a rectangular insulating support 130 onwhich five printed circuits are photoengraved. A first circuit 132extends along the longitudinal axis of symmetry of the support 130 andis formed by a linear strip conductive 134 and an output contact 136.The strip 134 and the contact 136 serve as a return collector for thepotentiometer. Two other printed circuits 138 and 140 are symmetricallylocated relative to the longitudinal axis of the base, each of thesecircuits including a transverse end 143', 144, a junction strip 146, andan output contact 140. Finally, two double circuits 154 and 156 arelocated adjacent opposite respective edges of the support 130*. Each ofthese double circuits 154 and 156 includes a respective output contact158 and 160 connected by two junction strips 162, 164 and 166, 168,respectively, to two contact ends 170, 172 and 174, 176, respectively.

This embodiment makes it possible to select different contacts whichwill provide linear resistance elements of different relative lengthsand hence a linear potentiometer of variable length. If a relativelylong potentiometer length is required, such as 80 mm., the element maybe mounted and connected between the contact ends 170 and 142. For apotentiometer of shorter length, for example 60 mm., the resistanceelement may be connected between the ends 172 and 142, for a stillshorter length of, for example 40 mm., the resistance element may beconnected between the ends 174 and 144 and for a relatively shortpotentiometer length, such as mm., the resistance element may beconnected between the ends 176 and 144.

It will be noted that, in this embodiment, each contact end and itsheating area form a continuous rectangular plate. In such apotentiometer, the wiper has a linear movement parallel to thelongitudinal axis of the base 130, this wiper insuring an electricalconnection between the resistance element and the collector strip 134.It will also be appreciated that the same advantages discussedhereinabove in regard to circular potentiometers are also obtained inpotentiometers of the linear type.

What is claimed is:

1. A potentiometer comprising, a support having one surface formed of anelectrically insulating material, an elongated, helical resistanceelement mounted adjacent the one support surface, at least oneelectrically conductive strip mounted on said one surface and having twoends, one end of said strip extending between said one support surfaceand a portion of said resistance element, a terminal electricallyconnected by soldering material to the other end of said strip and saidone end of said strip being connected by a soldering material to saidresistance element and a thermally conductive plate mounted on saidsupport surface adjoining said strip for supporting at least a portionof a heated soldering tool, said plate when heated thereby conductingheat to said strip and being spaced different distances from the twostrip ends such that the soldered connection of the more remote stripend remains intact when the temperature of the less remote strip end isincreased to that required for soldering.

2. A potentiometer comprising, a housing, a support member mounted onsaid housing and having at least one face thereof composed of anelectrical insulating material, an elongated helical resistance elementmounted adjacent the one support face, at least one circuit comprised ofa continuous, elongated, electrically conductive strip printed on saidone support face and having at least two ends, one end of said circuitextending between a portion of said resistance element and said onesupport face, whereby said one end of said circuit may be soldered to anoverlying portion of said resistance element, an elongated collectorcircuit printed on said one support face and extending substantiallyparallel to an adjacent portion of said resistance element, a wipermember engaging said resistance element and said collector circuit andproviding an electrical circuit therebetween, means fordisplacing saidwiper member relative to said resistance element and said collectorcircuit, a terminal soldered to the other end of said conductive strip,and a thermally conductive pad on said one support face joined in heatconducting relationship to said strip for receiving a heated solderingtool, said pad being located different predetermined distances from eachend of said strip to prevent the unsoldering of said terminal from theother strip end by heat conducted thereto when the one strip end isbeing soldered to said resistance element.

3. A potentiometer comprising, a housing; an insulating support mountedon said housing; an elongated helical resistance element mountedadjacent one face of said support and having two ends and a longitudinalaxis; the one support face having at least two electrical circuitsprinted thereon; each of said printed circuits including, an elongatedcontact strip having one end thereof extending between the one supportface and a different turn of said resistance element, each of saidcircuits further comprising, an elongated junction strip forindividually electrically connecting a terminal to a different one ofthe contact strips, each terminal being soldered to one end of ajunction strip remote from a corresponding one of the contact strips, acollector mounted on said one support face and having an axissubstantially parallel to the longitudinal axis of said resistanceelement; a wiper arm movable upon said resistance element and saidcollector in a direction parallel to the axes thereof, for establishingan electrical circuit therebetween, means for displacing said wiper armrelative to said resistance element and said collector and a pluralityof thermally-conductive pads having surfaces shaped to conformsubstantially to at least a portion of a heated soldering tool, each ofsaid pads joining a portion of a contact strip remote from said one endthereof to a portion of a corresponding junction strip and beingsufficiently remote on a heat transfer basis from a terminal soldered toa corresponding junction strip to prevent the inadvertent unsoldering ofsuch terminal by heat transmitted thereto from said pad during theapplication of heated and molten solder to a corresponding contactstrip.

4. The potentiometer according to claim 3 wherein the contact strips areof ribbon-like shape and include substantially coextensive, laterallyspaced-apart edges for accurately defining the length of the electricalpath between different turns of said resistance element to which thecontact strip edges are soldered.

5. The potentiometer according to claim 4 wherein the portions of saidjunction strips joined to said contact strips are of ribbon-like shapeand have edges in coalignment with corresponding ones of said contactstrip edges.

6. The potentiometer according to claim 5 wherein said v 7 8 portions ofsaid junction strips and the one ends thereof 2,842,647 7/1958 Khouri338-163 connected to different ones of the terminals are electrical-3,132,321 5/ 1964 Kuhlman 338-174 1y connected by individualribbon-like'elernents extend- 3,271,721 9/1966 Gordon 338-180 XR inglaterally from individual ones of the junction strip 3,354,418 11/1967Casey et al. 338174 portions in a direction away from the spacingdefined by a 5 3,358,259 12/ 1967 Kirkendall 338183 corresponding one ofsaid junction strip edges.

THOMAS J. KOZMA, Primary Examiner References Cited UNITED STATES PATENTS1,877,926 9/ 1932 Mattern 338-174 XR 10 2,266,188 12/1941 Foley et a1338168 XR

